Ball Mill Seal

Detailed Introduction to Ball Mill Sealing Rings and Their Manufacturing & Inspection Processes

I. Functions and Structural Features of Ball Mill Sealing Rings

• By sealing principle:

• Contact sealing rings: Achieve sealing through close contact between elastic materials (e.g., rubber, polyurethane) and the hollow shaft surface. Suitable for low-speed (≤30r/min) and low-dust conditions.

• Non-contact sealing rings: Use labyrinth structures (tortuous gaps between metal rings and hollow shafts) to block leakage via fluid resistance. Ideal for high-speed, high-temperature environments or scenarios where grinding media easily wear seals (e.g., large mining ball mills).

• Combined sealing rings: Integrate contact and non-contact features (e.g., "labyrinth + rubber lip" structure), balancing sealing effectiveness and wear resistance. Most widely applied.

• By material:

• Metal sealing rings: Typically made of gray cast iron (HT250), ductile iron (QT500-7), or stainless steel (304), used in labyrinth structures.

• Non-metal sealing rings: Rubber (nitrile rubber, fluororubber), polyurethane, or graphite, used in contact sealing. They must resist oil and temperature (-20℃~120℃).

• Key performance requirements:

• Sealing reliability: No significant leakage (≤5mL in 24 hours);

• Wear resistance: Wear on parts in contact with the hollow shaft ≤0.1mm/1000 hours;

• Adaptability: Fit clearance with the hollow shaft (interference of 0.1-0.3mm for contact types, clearance of 0.5-1.5mm for non-contact types).

II. Manufacturing Process of Ball Mill Sealing Rings

1. Manufacturing of Metal Skeleton (Gray Cast Iron HT250)

• Casting of blanks:

• Mold design: Sand casting molds with a machining allowance of 3-5mm and risers to prevent shrinkage cavities;

• Melting and pouring: Control of molten iron composition (C 3.1-3.4%, Si 1.8-2.2%, Mn 0.8-1.0%) and pouring temperature (1350-1400℃);

• Aging treatment: Natural aging for 3 weeks (or artificial aging at 200℃ for 8h) to eliminate casting stress.

• Machining:

• Rough turning: Lathe machining of outer circle and end faces, leaving 1-2mm for finish turning;

• Finish turning: CNC lathe machining of sealing mating surfaces (grooves for rubber bonding, labyrinth gaps) with surface roughness Ra≤3.2μm and inner diameter tolerance H8;

• Drilling: Machining of mounting bolt holes (evenly distributed circumferentially, position tolerance ±0.1mm).

2. Manufacturing of Rubber Sealing Lip (Nitrile Rubber NBR)

• Compounding and mixing: Mixing nitrile rubber (70-80 parts) + carbon black (20-30 parts) + vulcanizing agent (sulfur 2-3 parts) in an internal mixer at 120-140℃ for 10-15 minutes to ensure uniform dispersion;

• Vulcanization molding:

• Preheating the mold to 160-170℃ and positioning the metal skeleton in the mold;

• Injecting rubber compound, vulcanizing under pressure (10-15MPa) for 15-20 minutes, and bonding rubber to the metal skeleton firmly with an adhesive (e.g., Chemlok 205);

• Trimming: Removing flash and burrs to ensure a flat sealing lip (thickness deviation ≤0.1mm).

3. Assembly and Surface Treatment

• Metal parts: Sandblasting non-mating surfaces for rust removal, then applying anti-rust paint (thickness ≥60μm);

• Combined assembly: Checking the bonding strength between the rubber lip and metal skeleton (peel strength ≥5N/cm) and installing a spring ring (to enhance lip pressure);

• Final trimming: Test-fitting the sealing lip with a dummy shaft to ensure uniform contact (checked with red lead powder, contact area ≥90%).

III. Inspection Process of Ball Mill Sealing Rings

1. Raw Material Inspection

• Metal materials:

• Cast iron parts: Spectral analysis for composition verification, tensile testing for tensile strength ≥250MPa, and hardness 180-240HBW;

• Rubber: Infrared spectroscopy to confirm nitrile rubber purity (acrylonitrile content 30-40%), oil resistance test (immersion in L-TSA46 turbine oil for 24h, volume change rate ≤10%).

• Blank defect inspection:

• Cast iron parts: Magnetic particle testing (MT) for surface cracks (no defects ≥0.5mm allowed), penetrant testing (PT) for pores (depth ≤1mm).

2. In-Process Inspection

• Metal skeleton machining:

• Dimensional accuracy: CMM measurement of inner diameter (tolerance H8) and sealing groove depth (±0.05mm);

• Geometric tolerance: End runout ≤0.05mm, radial circular runout ≤0.03mm (measured with a roundness meter).

• Rubber sealing lip:

• Hardness: Shore A hardness 65±5 degrees (ensuring moderate elasticity);

• Vulcanization quality: Section inspection for no bubbles or missing rubber, and no delamination at the rubber-metal bonding interface.

3. Finished Product Inspection

• Sealing performance test:

• Static pressure test: Installing on a dummy shaft, introducing 0.3MPa compressed air (or lubricating oil), and holding pressure for 30min with no leakage;

• Dynamic test: Running on a test bench at 30r/min for 100h, measuring leakage (≤5mL) and sealing lip wear (≤0.05mm).

• Assembly dimensions:

• Fit between inner diameter and hollow shaft: Interference of 0.1-0.3mm for contact types (checked with feeler gauges), labyrinth gap of 0.5-1.5mm for non-contact types (measured with a dial indicator);

• Position accuracy of mounting holes: Checking coaxiality between bolt holes and locating spigots with a gauge (≤0.1mm).

• Appearance and packaging:

• Rubber surfaces free of scratches and bubbles, metal parts free of rust;

• Applying anti-rust oil before packaging and fixing with wooden cases (to prevent deformation during transportation).